timing: Add TimingAnalysis::get_clock_skew_paths

[rowanG077]

Which is useful if you want to optimize clock skews and need the FFs which are also affected if you change a single clock delay by re-routing

[rowanG077]

@Ravenslofty Please check if you are oke with this API. You still have to calculate the skew yourself per pair. But since if you have let's say 100 startpoints and 100 endpoints then there are 100*100 clock skew values. So I don't think it makes sense to store this in the struct itself. You can recover the skew pretty easily by just subtracting the clock delays, which are given.

[Ravenslofty]

I'm gonna have to think on this a bit; it's a little too late in the day for me to picture how this will work.